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Measuring Molecular Elasticity by Atomic Force Microscope Cantilever Fluctuations

Bryan T. Marshall ^*, Krishna K. Sarangapani , Jianhua Wu ^*, Michael B. Lawrence , Rodger P. McEver  ^¶ and Cheng Zhu ^* 

^* George W. Woodruff School of Mechanical Engineering, and Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology, Atlanta, Georgia; Department of Biomedical Engineering, School of Medicine and School of Engineering and Applied Science, University of Virginia, Charlottesville, Virginia; and Cardiovascular Biology Research Program, Oklahoma Medical Research Foundation, and ^¶ Department of Biochemistry and Molecular Biology and Oklahoma Center for Medical Glycobiology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma

Correspondence: Address reprint requests to Dr. Cheng Zhu, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0363. E-mail: cheng.zhu{at}me.gatech.edu.

In single-molecule mechanics experiments the molecular elasticity is usually measured from the deformation in response to a controlled applied force, e.g., via an atomic force microscope cantilever. We have tested the validity of an alternative method based on a recently developed theory. The concept is to measure the change in thermal fluctuations of the cantilever tip with and without its coupling to a rigid surface via the molecule. The new method was demonstrated by its application to the elasticity measurements of L- and P-selectin complexed with P-selectin glycoprotein ligand-1 or their respective antibodies, which showed values comparable to those measured from the slope of the force-extension curve. L- and P-selectin were found to behave as nearly linear springs capable of sustaining large forces and strains without sudden unfolding. The measured spring constants of 4 and 1 pN/nm for L- and P-selectin, respectively, suggest that a physiological force of 100 pN would result in an 200% strain for the respective selectins.

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